remember that you'll pay sales tax in most states in the US, so when you're comparing prices don't forget to add that amount to the purchase price and any Canadian taxes that you'll have to pay when you take it into Canada

1) I will need to pay the state tax in US (say, 7% in NY?)
2) and I still need to pay the GST in British Columbia, Canada
3) Do i need to pay PST (cuz its out of the province?)??
4) what custom/duty does a laptop apply?

Ok the slightly dishonest way:
Buy it in Seattle, then take it out of the packaging, send the packaging empty via post to yourself (can't apply computer tax on an empty box). make sure your bill is hidden on your person. Take your compy across the border and if asked you brought it with you when you crossed over before
DO NOT SHOW THEM THE BILL - IT IS A DEAD GIVEAWAY
OR you can be legit and buy from apple.ca, they are good, I've bought my ibook and an intel imac from them.

lasty item I got something from the us the duty was around 5%? and they also charge you gst, pst etc.

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